The present invention relates to a process for preparing an acrylic polymer having an excellent water solubility and a high molecular weight, and more particularly to a process for preparing a particulate water-soluble high molecular weight polymer having an excellent water-solubility and a uniform particle size by photopolymerization.
Water-soluble acrylic polymers, particularly acrylamide polymers, have been widely utilized as paper sizing agent, viscosity builder, waste water treating agent, precipitant for ore, etc. In particular, the use as a flocculant for various industrial waste waters is increasing more and more in recent years as a countermeasure for the environmental pollution. It has been generally considered that the properties of water-soluble polymers required for flocculant use are proportional to the molecular weight, and accordingly water-soluble polymers having a higher molecular weight tend to be demanded more and more.
Water-soluble acrylic polymers have been prepared by bulk polymerization, suspension polymerization, emulsion polymerization or solution polymerization. In general, high molecular weight polymers usable as a flocculant have been prepared by polymerization of an aqueous solution of a monomer.
The most general process for the preparation of high molecular weight water-soluble polymers in an aqueous solution system is a polymerization using a polymerization initiator producible a radical by heat energy or a peroxide-reducing agent redox initiator, and wherein the polymerization is carried out under mild conditions, e.g. at a relatively low concentration of a monomer and at as low temperatures as possible.
Other polymerization processes not using heat energy are a process using a light energy such as ultraviolet rays or visible rays, a process using a radiation energy, a process wherein the polymerization is conducted under high pressure, and the like. For instance, Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 2094/1971 discloses a process for preparing a water-soluble polymer by subjecting an aqueous solution of an ethylenically unsaturated monomer to radiation polymerization.
However, the light- or radiation-initiated polymerization is liable to produce a polymer rich in a three dimensional network structure, namely a water-insoluble polymer, because the energy thereof is very large and the polymerization proceeds at a high rate, whereby occurrence of graft polymerization is accelerated. Also, even if a polymer having a little network structure is obtained, the molecular weight is very low and the polymer is not suited at all for uses requiring a high molecular weight, for instance, as flocculant. It is the present situation that the light- or radiation-initiated polymerization method is scarcely adopted on an industrial scale in production of water-soluble polymers having a high molecular weight for the reason mentioned above.
On the other hand, the light- or radiation-initiated polymerization is advantageous in that the polymerization proceeds at a very high rate. For instance, in case of polymerizing an aqueous solution of a monomer into which a polymerization inhibitor is incorporated as usually done, but in a larger amount, the polymerization is not initiated with ease by heat energy, but can be initiated in a very short period of time by irradiation of a light. In particular, in view of easy availability of a light energy source at a low cost in recent years, the utilization of light energy is desired in industrial production of water-soluble high molecular weight polymers.
Also, the light-initiated polymerization has some advantages as compared with conventional polymerization methods using heat energy. For instance, since the polymerization rate is very high, the polymerization time is markedly shortened and accordingly the productivity is improved. In view of the latest attention drawn to the toxity of monomers, a high conversion of monomers leads to lightening of environmental pollution. Also, from a viewpoint of installations, they can be made compact, since continuous polymerization in short time is possible. Accordingly, development of a technique which is improved in light-initiated polymerization so as to control or prevent the formation of three dimensional network structure and moreover which can produce polymers having a high molecular weight would greatly contribute to industrial progress.
It is an object of the present invention to provide a process for preparing water-soluble acrylic polymers which have a very high molecular weight and moreover have an excellent solubility in water, with good productivity by using a photopolymerization technique on an industrial scale.
A further object of the present invention is to provide a process for preparing a water-soluble high molecular weight polymer powder having a uniform particle size and an excellent water-solubility.
These and other objects of the present invention will become apparent from the description hereinafter.